Catalytic dehydrogenation of hychocarbons has been carried out for many years and constitutes an important catalytic process in view of the increasing demand for dehydrogenated products which may be utilized in their many various forms in products such as high octane gasolines, plastic materials and synthetic rubbers.
In the field of dehydrogenation of alkylaromatic hydrocarbons, known processes include thermal dehydrogenation, catalytic dehydrogenation in the presence of an inert diluent such as steam, and oxidative dehydrogenation, the latter involving the injection of molecular oxygen into the reaction medium. Although oxidative dehydrogenation may have the same advantages regarding reaction yield and selectivity of the desired product, it is also well known that the presence of molecular oxygen in the reaction medium leads to the formation of undesirable oxidation products such as aldehydes.
In order to partially obviate these drawbacks, it has been proposed to use very specific catalysts having a particular selectivity towards oxidation dehydrogenation of certain hydrocarbons whether or not of the alkylaromatic type.
In this respect, U.S. Pat. No. 4,777,319 to Kung, et al. teaches the use of vanadates or molybdenates for the selective dehydrogenation of paraffinic hydrocarbons having from 3 to 6 carbon atoms. However, the dehydrogenation reaction must be carried out in the presence of molecular oxygen, which displaces the thermodynamic equilibrium but leads to the formation of undesirable secondary oxidation products.
It has also been proposed in U.S. Pat. No. 4,742,180 to Gaffney to use supported catalysts, the support being essentially an oxide of praseodymium on which there is deposited an alkaline metal having a dehydrogenation action. However, the limited availability of praseodymium oxide for the production of huge amounts of alkenylaromatic hydrocarbons is a distinct disadvantage. Moreover, it should be noted that no significant result is indicated for the dehydrogenation of alkylaromatic hydrocarbons. Oganowski Bulletin Polish Acad. Sci., Chemistry 31 (1983), pp. 129, 139, 153; J. Mol. Catal. 29 (1985) 109.! has also proposed use a catalyst of the Mg.sub.3 (VO.sub.4).sub.2 type for the dehydrogenation of ethylbenzene, but the reaction must occur in the presence of a gas containing molecular oxygen. Moreover, it is known that oxides of the V.sub.2 O.sub.5 type lead to a reaction of complete combustion of the hydrocarbon feed to CO.sub.2 and H.sub.2 when used in the dehydrogenation reaction of hydrocarbons in the absence of molecular oxygen. This results in loss of production (low selectivity) and lowered yields. See U.S. Pat. No. 4,816,243 to Bradzil et al.!
An object of the present invention is to provide an improved process for the catalytic dehydrogenation of alkylaromatic hydrocarbons in the presence of a redox catalytic system.
Another object of the present invention is to provide an improved process for the catalytic dehydrogenation of alkylaromatic hydrocarbons in the absence of molecular oxygen.
A further object of the present invention is to provide an improved process for the catalytic dehydrogenation of alkylaromatic hydrocarbons in the presence of a redox catalytic system, whether or not supported, and of which one or more oxidation stages show a dehydrogenating activity.